1. Field of the Invention
The invention relates to a discharge lamp, especially a mercury lamp of the short arc type which is used to expose a semiconductor wafer and the like.
2. Description of the Prior Art
In a mercury lamp of the short arc type for a reduction projection exposure device of the stepper type which exposes a semiconductor wafer, mercury and an inert gas are encapsulated in an emission space which also contains a cathode and an anode opposite one another. Ultraviolet radiation is used which is emitted from the emission space. Hereinafter, this mercury lamp of the short arc type is called simply a lamp, for short. Conventionally, a g-line with a wavelength of 436 nm is mostly used. Recently, however, an i-line with 365 nm has been used according to miniaturization of the semiconductor.
Two disadvantages have been ascertained recently with respect to this lamp:
The first disadvantage is that ultraviolet radiation with short wavelengths is formed. However, it is necessary that no short-wave ultraviolet radiation be emitted since it reacts with the oxygen in the air, and thus, produces harmful ozone, and because it acts directly on the human skin and therefore is undesirable. If ultraviolet radiation of less than or equal to 200 nm is cut, which can also be called "filtering out", production of ozone can for the most part be prevented. To completely cut the ultraviolet radiation of less than or equal to 200 nm, there is a need for an absorption characteristic by which at least light with 230 nm is cut by at least 50%.
As processes for cutting of this short-wave ultraviolet radiation, there are conventional examples in which quartz glass is coated with titanium oxide (TiO.sub.2), or in which quartz glass which is doped with TiO.sub.2 is used as an arc tube. Recently, there has, furthermore, been a demand for cutting ultraviolet radiation with long waves.
A second problem is to increase the amount of light with 365 nm which is emitted from the lamp. Recently, in the semiconductor industry, exposure surfaces have been increasing in size in order to increase throughput in exposure processes. Furthermore, there is the tendency here that the required amount of ultraviolet light radiation per unit of surface area increases. Therefore, there is a need for a lamp with high power. However, such a lamp also has high heating power, for which reason a cooling device for keeping the temperature constant within the stepper also becomes very large and complicated. Consequently, it is a major problem to increase the amount of light with 365 nm while keeping the lamp power constant.
With regard to the first problem of cutting the short-wave ultraviolet radiation, 1) a process in which so-called doped quartz glass (or ozone-free quartz glass) is used, in which titanium oxide is mixed and dispersed into the above described quartz glass, 2) a process in which a diffusion layer of titanium oxide is applied to the inner surface of the arc tube by dipping and then burned in, as is known from Japanese patent HEI 4-34258, 3) a process in which the outer surface of the arc tube is coated with a glass layer which contains titanium dioxide and silicon oxide (SiO.sub.2), as is known from the Japanese patent HEI 5-70901, or similar processes have been reported.
In these processes, ultraviolet radiation of less than or equal to 200 nm can be cut by roughly 100% and the ultraviolet radiation of 230 nm by roughly 50%. As was described above, for complete cutting of the ultraviolet rays of a wavelength less than or equal to 200 nm, it is necessary, with respect to the ozone, to cut at least 50% of the ultraviolet radiation of less than or equal to 230 nm. It is, however, more desirable to be able to cut ultraviolet radiation with wavelengths of less than 230 nm without cutting the ultraviolet radiation of 365 nm wavelength which is necessary for exposure.
The second problem, i.e., the attempt to increase the amount of light with 365 nm, was conventionally approached by optimizing the encapsulation amounts of mercury and rare gas, by optimizing the shapes of the anode and the cathode, and by similar methods.
Conventionally, the first and the second problems were solved separately. However, these processes can only solve one of the problems. Here, therefore, the two problems could not be solved at the same time.
It is also noted that a so-called multilayer anti-reflecting coating is known in which, by alternating a plurality of coating layers containing Ta.sub.2 O.sub.5 with other coating layers on top of one another, a reflection characteristic for certain wavelengths is obtained.
With this multilayer anti-reflection coating, to increase the reflection factor of a certain wavelength (.lambda.), coating layers of a high refraction coefficient (for example, TiO.sub.2 with a refraction coefficient from 2.2 to 2.7 or Ta.sub.2 O.sub.5 with a refraction coefficient from 2.2 to 2.4) and with coating layers with a low refraction coefficient (for example, SiO.sub.2 with a refraction coefficient of 1.475 or MgF.sub.2 with a refraction coefficient of 1.38), as the optical layer thickness (nd), coating in a thickness of 1/4*.lambda. is performed in an alternating manner and a plurality of these layers are placed on top of one another.
In this technique, the desired reflection factor cannot be obtained with a small number of layers placed on top of one another. To obtain an essentially practicable reflection factor, coating conventionally had to be done with at least ten layers.
For example, Japanese patent HEI 2-18856 discloses a super-high pressure mercury lamp which is used for a stepper. Here, the technique of multilayer coating is described in which infrared light is reflected within the lamp in order to dispense with a filter or a cold mirror in the optical system.
Specifically, for reflecting light with a wavelength of 1000 nm with Ta.sub.2 O.sub.5 and SiO.sub.2 in a thickness of 250 nm, twelve layers of alternating coating are used and layers with a high refraction coefficient (Ta.sub.2 O.sub.5) with a thickness of 250 nm and layers with a low refraction coefficient (SiO.sub.2) with a thickness of 125 nm are placed on top of one another. Consequently, light with wavelengths of 800 to 1100 nm is reflected essentially within the lamp, and thus, a transmission factor of 0% is obtained.
Furthermore, Japanese patent HEI 7-19568 discloses a technique in which a hemispherical surface of a lamp is provided with a plurality of reflecting layers for reflecting light with a wavelength of 365 nm and in which light with a 365 nm wavelength is radiated from one half surface.
Specifically, a coating is produced with nine layers of Ta.sub.2 O.sub.5 and SiO.sub.2 with a thickness of 90 nm, with a layer thereof of a thickness of 100 nm, and then, with nine layers thereof with a thickness of 110 nm, and mainly light with a wavelength of 400 nm is reflected. It is known therefrom that essentially 100% of the light around 365 nm is reflected as a result.
In this prior art the desired light is reflected from the light which is emitted from the emission center in the arc tube, by mutual light interference as a result of the plurality of coating layers provided on the outer surface of the arc tube. Furthermore, an attempt is made here to transmit the desired light without losses by reflection while the undesired light is attentuated by reflection.